In many cases, machine tools in which the kinematic control of a tool and a worktable is executed by a numerical control apparatus (CNC) include at least one pair of gears for changing speeds or rotational directions, arranged in a transmission line through which the rotating force of a servomotor is transmitted to a driven member such as the worktable.
If, however, even one pair of gears is provided in the transmission line for transmitting the rotating force of the servomotor to the driven member, pitch errors in the gears affect the kinematic control of the driven member, and thus the positioning accuracy is lowered.
Therefore, a gear pitch error correction is conventionally executed in numerical control apparatuses for machine tools including at least one pair of gears arranged in the transmission line, via which the rotating force of a servomotor is transmitted to a driven member. This gear pitch error correction is carried out, for example, on the basis of error amounts each previously measured in units of predetermined angles as a value between an actual amount of rotation of a rotary table and a target amount of rotation, when a force corresponding to a rotation of the predetermined angle has been applied to the rotary table as the driven member by the servomotor.
To carry out a precise correction by the above-mentioned gear pitch error correction procedure, however, the rotational angle pitch for measuring each error amount must be made smaller, to thereby permit a large number of correction points to be used for the pitch error correction, and therefore, correction value data must be prepared for each of many correction points. Accordingly, to enhance the precision of the pitch error correction, an increased amount of correction value data is required, and thus the capacity of a memory for storing the correction value data must be increased.
To eliminate the above drawbacks of the conventional gear pitch error correction, the inventor made an intensive study of the gear pitch error correction procedure, and as a result, found that, since the gear pitch error is caused by pitch errors and shape errors of each gear, a precise gear pitch error correction can be made by determining the gear pitch errors of the individual gears and then superimposing these errors, to thus eliminate the need for the use of a large amount of correction value data associated with numerous correction points.